Admittedly, it has spent half a series doing the TV equivalent of moving around the furniture, but in recent weeks Being Human has arguably returned to its best, with the new line-up bedding in well. Just in case that wasn't the case, however, BBC3's hit horror comedy has been keeping one last trump card up its sleeve. We saw his arrival trailed in a red-button clip in the first week, but this Sunday, Mark Gatiss arrives as Mr Snow, vampiric leader of the fabled Old Ones.

Gatiss seems to be ticking off a checklist of roles in BBC cult dramas. "I'm cementing my future on the convention circuit!" he quips to the Guardian, but, having worked alongside creator Toby Whithouse on Doctor Who since 2006, Gatiss said yes to the Being Human role immediately. "There was a point when we were filming where I thought to myself, 'I may have waited all my life to play this part'. When I was a kid I used to recreate the end of Dracula, rising from the grave. All these years I've wanted to be king of the vampires, so I had a fantastic time."

Not that any of his work looks like a terrible drag: on his to-do list are some more Doctor Who episodes, plus the small matter of the third series of Sherlock which, executive producer Beryl Vertue announced yesterday, will begin filming in early 2013.

Earlier this year Steven Moffat suggested that feverish fan theories as to how Holmes had faked his own death had missed one vital clue. So does Gatiss think the truth could now be out there? "There's some very clever theories, some of them elaborate, and I enjoy them all. But if I were to tell you if someone had worked it out then it wouldn't be a secret." Which, of course, isn't an answer. So I try again. Has somebody somewhere now worked it out? "It may be, sort of, in some of the theories. There's a lot of very clever people out there … "

Gatiss points to certain theories beyond his wildest imagination, and admits to being happily shocked by the frenzy that surrounded Holmes' jump. "I've never known something become such a public talking point." And one that shows little sign of abating. "It'll be worth the wait," he promises.

On the planned US Sherlock Holmes update, starring Johnny Lee Miller and Lucy Liu, he pauses for a moment. "All I can say … is no comment."

Gatiss also looks set to make quite an impact in Being Human. As the leader of the Old Ones, he plays the biggest baddie the show has yet witnessed. "There's nothing like having a good build-up! Toby sent me the script, and at the end of episode seven, when we first arrive, the stage direction is something like 'dark as night and older than dirt!'," he says with glee. "He's so old he doesn't even have a name, so they just call him Mr Snow. It's perfect!"

Being Human has always revelled in a kitchen sink brand of supernature. Only now with the stakes this high can they let fly four years of pent-up camp. And Gatiss relishes in every lip-smacking second.

The arrival of the Old Ones has been the major story arc in this year's Being Human, as they sail in a shipping container to the deliciously named Stoker Imports And Exports on the coast of Barry. And the reason they're starting their incursion there? The show's new resident vampire, OCD recovering psychopath, Hal, played by Damien Molony. "The Old Ones have come back to pick up where they left off with Hal," Gatiss explains. "Hal seems to have escaped them for the past 55 years, and then there's a lovely scene where I say to him I've just give him the afternoon off. Because the timescale that we work on, 55 years is just the blink of an eye."

Between Sherlock and Doctor Who, Gatiss has made a career out of indulging his boyhood obsessions. "Vampires are so incredibly popular at the moment, I think in years to come people will look back on this period, with all the films and TV series, as like the end of the 19th century when they first came to prominence in a kind of Byronic way. They're everywhere at the moment, we all have a collective fear that we've internalised, of being drained. I don't know why, maybe it's bankers."

With that in mind, Gatiss had very specific ideas about what he wanted to bring to the vampire party. "I wanted to do something different with it. Before Dracula they weren't these gothic, Byronic, romantic creatures, they were filthy and disgusting.

"I wanted to have red hair, because you never have ginger vampires, and terrible teeth, and these really dirty fingernails. And they gave me everything I wanted. So there's lots of lovely close-ups of my filthy hands and terrible teeth. He's about 3000 years old, he's literally rotting from the inside."

In fact there was one compromise made. "They asked me if I'd shave my head, and though I've done it before, I really wanted the red hair. But if I came back then I'd shave my head. The full Nosferatu!"